Channel assignment in a spread spectrum CDMA communication system

ABSTRACT

A selected user equipment transmits a signature in a selected one of the common packet channel&#39;s time slots. The base station, in response to receiving the transmitted signature, selects a currently unused code, if available, out of a plurality of code associated with the access opportunity defined by the selected signature and selected time slot. The base station transmits an acknowledgment signal comprising an identifier of the selected code. The selected user equipment receives the acknowledgment signal. The selected user equipment and the base station communicate using the selected code.

BACKGROUND

The invention relates generally to resource allocation in a wirelesscode division multiple access communication system. More specifically,the invention relates to assigning uplink and downlink channels inresponse to access requests of user equipment.

FIG. 1 depicts a wireless spread spectrum Code Division Multiple Access(CDMA) communication system 18. A base station 20 communicates with userequipment (UE) 22-26 in its operating area. In a spread spectrum CDMAsystem 18, data signals are communicated between UEs 22-26 and the basestation 20 over the same spread bandwidth. Each data signal in theshared bandwidth is spread with a unique chip code sequence. Uponreception, using a replica of the chip code sequence, a particular datasignal is recovered.

Since signals are distinguished by their chip code sequences (code),separate dedicated communication channels are created using differentcodes. Signals from the base station 20 to the UEs 22-26 are sent ondownlink channels and signals from the UEs 22-26 to the base station 20are sent on uplink channels. For coherent detection of downlinktransmissions by the UEs 22-26, pilot signals are transmitted to all theUEs 22-26 within the base station's operating range. The UEs 22-26condition their receivers based on the pilot signals to enable datareception.

In many CDMA systems, a common packet channel (CPCH) is used for uplinktransmissions. A CPCH is capable of carrying packets of data fromdifferent UEs 22-26. Each packet is distinguishable by its code. Fordetection by the base station 20, the packets have a preamble which alsodistinguishes it from other packets. The CPCH is typically used to carryinfrequently communicated data at high rates.

FIG. 2 depicts a CPCH access scheme 28. The CPCH access scheme 28 istime divided into intervals having time slots 30-34, such as 8 timeslots proposed for the Third Generation Mobile Telecommunications System(IMT-2000). A group of predetermined signatures 3640 are assigned to thetime slots 30-34 to allow more than one UE 22-26 to use the same timeslot 30-34. A particular signature used within a particular time slot isreferred to as an access opportunity 66-82. For instance, for each ofthe 8 time slots in the proposal for IMT-2000, one out of 16 signaturesis available to be chosen, resulting in 128 access opportunities. Eachsignature 36-40 is preassigned a virtual channel. A virtual channeluniquely defines operating parameters for both the uplink and downlink,i.e., an uplink spreading factor and a unique code for the downlink.

Broadcast from the base station 20 to each UE 22-26 is the availabilityof each virtual channel over an acknowledge indication channel (AICH).The UE 22-26 monitors the AICH to determine the availability of eachvirtual channel. Based on the operating parameters required by the UE22-26 and the availability of the virtual channels, the UE determinesthe access opportunity to select. Upon identifying a particular accessopportunity, the base station 20 sends out an acknowledgment message(ACK) if the corresponding downlink channel is still available. In theproposal for IMT-2000, the ACK simply repeats the signatures 3640associated with the access attempt. If the downlink channel is notavailable, a negative acknowledge (NAK) is sent.

After receiving a corresponding acknowledgment, the UEs 22-26 determinethe proper code to recover communications on the downlink channel basedon the access opportunity 66-82 used to send the UEs' packet. Eitherstored in the UEs 22-26 or transmitted on a Base Station's BroadcastChannel is a list of the code assigned to each access opportunity 66-82.This scheme severely increases the packet collision probability andtherefore the packet delay which is undesirable.

In some situations, monitoring the AICH is not desirable. At aparticular moment, some UEs 22-26 will be operating in a “sleep” mode.In the “sleep” mode, the UE 22-26 only runs when there is a need to senddata. Monitoring the AICH during “sleep” mode will both reduce batterylife and introduce a delay in the transmission of the first packet.Additionally, when a UE 22-26 borders between two base station'soperating areas, monitoring multiple AICHs further exacerbates thesedrawbacks.

Monitoring creates other problems. It further complicates the UE'sreceiving circuitry, making the UE 22-26 more expensive.

Monitoring results in a suboptimum use of the CPCH. AICH monitoringprovides information when a channel becomes busy. The time at which thechannel becomes free is deduced on a worst case maximum packet length.If a packet is not maximum length, the channel will be idle while theUEs 22-26 are waiting to transmit. On the other hand, if monitoring isnot performed in such a system, channel availability information isunavailable. The UE 22-26 may randomly choose a busy virtual channelincreasing the packet delay by causing a collision. Accordingly, itwould be desirable to allow the UEs 22-26 to wait a period shorter thanthe maximum packet length and provide for some other collision reducingmechanism.

One technique to reduce the possibility of collision is to raise thenumber of codes, for instance to 128 different codes. In the proposalfor IMT-2000 the 128 sequences represent approximately half of thesequences available at the base station 20. Accordingly, this solutionis undesirable. Additionally, since monitoring the AICH complicates theUE receiver circuitry increasing its cost, it is undesirable.Accordingly, an alternate approach to assign virtual channels isdesirable.

SUMMARY

A selected user equipment transmits a signature in a selected one of thecommon packet channel's time slots. The base station, in response toreceiving the transmitted signature, selects a currently unused code, ifavailable, out of a plurality of code associated with the accessopportunity defined by the selected signature and selected time slot.The base station transmits an acknowledgment signal comprising anidentifier of the selected code. The selected user equipment receivesthe acknowledgment signal. The selected user equipment and the basestation communicate using the selected code.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a typical wireless spread spectrum CDMAcommunication system.

FIG. 2 is an illustration of a common packet channel access scheme.

FIG. 3 is an illustration assigning virtual channels.

FIG. 4 is a graph of the probability of a collision versus demand forthe prior art and the virtual channel assignment.

FIG. 5 illustrates a simplified base station and user equipment.

FIG. 6 is an identifier transmitter circuit.

FIG. 7 is an identifier receiver circuit.

FIG. 8 is a table of an assignment of Golay sequences.

FIG. 9 is a circuit for detecting the Golay sequences of FIG. 8.

FIG. 10 is an assignment table for a system having physical channelswith two time slots.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments will be described with reference to thedrawing figures where like numerals represent like elements throughout.FIG. 3 illustrates a virtual channel assignment scheme. Each virtualchannel 48-64 is defined by its operating parameters, such as uplinkspreading factor and down link code. Additionally, instead of assigningvirtual channels 48-64, the same principles may be applied to assigningphysical channels which are defined by their downlink code.

To reduce the number of used physical channels and increase the powerlevel of each channel, each physical channel may be multiplexed, such asby using two time slots. Using two time slots will increase thechannel's effective data rate, such as from 8 Kbps to 16 Kbps. In such asystem, the virtual channel 48-64 also defines which multiplexed signalis assigned to the UE 22-26.

Instead of assigning a single virtual channel for each signature 36-40as in the prior art, a set 42-46 of virtual channels are assigned foreach grouping 116-120 of access opportunities. A grouping may containall of the channels in one group or as few as 2 or 3 channels. Onepossible virtual channel grouping may group all virtual channels withthe same data rate for the uplink. For groups having the same uplinkdata rate, the UE 22-26 selects an access opportunity out of the groupshaving the UE's desired uplink data rate. Another grouping may be formedbased on the access opportunities' signature 36-40. Based on theselected access request and the UE's priority, one of the virtualchannels 48-64 assigned to the group 116-120 associated with the accessattempt is used for the UE if available. Once the virtual channel isassigned, it will not be assigned again until the particular UE'stransaction is complete. Additionally, a receiving circuit at the basestation 20 with the proper data rate is assigned to the UE 22-26.

In the prior art system, the UEs 22-26 determine which channel isassigned to the downlink based on the access opportunity 66-82. Virtualchannel assignment transmits a channel identifier 84-88, preferablyalong with the ACK, indicating which of the set 42-46 of channelsassigned to the group 116-120 is selected. When all of the virtualchannels are in the same group, the identifier 84-88 indicates theselected virtual channel. If no channel is available out of the set42-46, a no channel is available (NAK) identifier is sent. Since morethan one virtual channel is potentially assigned to a particular accessattempt, the probability of UE collisions is reduced.

FIG. 4 is a graph 91 depicting the probability of a collision(Collisions) versus the number of UEs 22-26 requesting access (Demand).As shown, the collisions using 2 or 3 virtual channels per group (2states/AP or 3 states/AP) is lower than the prior art (AICH monitor)regardless of demand.

FIG. 5 illustrates a simplified base station 20 and a UE 22 for use inimplementing channel assignment. The UE 22 has a controller 144 fordetermining the code of the uplink and downlink communications. A UEtransmitter 140 sends communications, such as access opportunities anduplink packet signals, to the base station 20. A UE receiver 142receives communications, such as ACK messages, NAK messages and downlinksignals.

The base station 20 has a controller 134 for determining the code ofuplink and downlink communications as well as determining channelavailability. A base station transmitter 136 sends communications, suchas ACK messages, NAK messages and downlink signals, to the UE 22. Thebase station receiver 138 receives communications, such as accessopportunities and uplink packet signals.

Techniques for sending the identifiers are to attach extra bits to theACK or to change the phase of the ACK to indicate the selectedidentifier. For a system using a single group of virtual channels, theextra bits identify the selected virtual channel. Circuits for sendingthe identifiers by phase shifting the ACK are depicted in FIGS. 6 and 7.The circuits are capable of sending up to four channel identifierswithout a NAK identifier or three channel identifiers with a NAKidentifier. In the transmitter circuit 92 of FIG. 6, the ACK sequence isgenerated by a sequence generator 94. The sequence itself is associatedwith the preamble access opportunity and is unique to the accessattempt. Several such sequences may be transmitted to several users atthe same time. The ACK sequence is passed through a mixer 96 whichmultiplies the signal with either +1 or −1. The mixed signal issubsequently passed through another mixer 98 where the signal is mixedwith an in-phase carrier (cos wt) or a quadrature carrier (sin wt). As aresult of the two mixers 96, 98, the transmitted ACK is at one of fourphases 0°, 900, 180° or 270°. Each identifier 84-88 is preassigned toone of the phases.

The receiver circuit 14 of FIG. 7 is used to determine the phase of theACK sent by the transmitter circuit 92 of FIG. 6. The ACK is mixed withboth an in-phase carrier by mixer 100 and a quadrature carrier by mixer102. Each of the mixed signals are correlated with a replica of theACK's sequence by sequence correlators 104, 106. The in-phase andquadrature correlation signals are each negated by mixers 108, 110 bymultiplying the correlation signals by −1. The two correlated signalsand the two negated signals are supplied to an identifier circuit 112.The identifier circuit 112 determines which of the four phased versionsof the correlated signal has the highest magnitude. Since the downlinktransmissions from the base station are synchronized and their phase isknown, the identifier circuit 112 determines which identifier 84-88 wassent based on the phase of the ACK. A list stored either in the UEs22-26 or transmitted on a Base Station's Broadcast Channel is used todetermine the virtual channel 48-64 associated with the identifier 84-88and the group 116-120 of the UE's access request. Using the determinedvirtual channel 48-64, transmissions sent by the base station 20 usingthe selected downlink channel's code are recovered at the UE 22-26.

Another technique for sending the identifier 84-88 is to use the ACK anda collision resolution signal (CR). After a collision between UEs 22-26is detected at a base station 20, in many spread spectrum systems thebase station 20 sends a CR directed to the colliding UEs. The CR has asequence which is associated with a specific UE 22 for detection by theUE 22. By inverting the ACK and CR, an identifier 84-88 is sent to thespecific UE 22. An inverted ACK indicates a NAK. By inverting the CR,one virtual channel is assigned to +CR a second virtual channel isassigned to −CR. Accordingly, using the ACK and CR an identifierindicating either a NAK or one of two channels is sent. Additionally,using a CR with multiple states, such as three, one of multiple channelsis assigned to the CR.

Alternatively, the identifier is sent with a signal using a Golaysequence. A Golay sequence is constructed out of short sequences, suchas X and Y. By inverting the shorter sequences and changing their ordermany unique longer sequences may be constructed as shown in table 122 ofFIG. 8. To reduce the size of the table 122, only half of the possiblesequences are shown. By negating each sequence, another unique Golaysequence results. As shown in FIG. 8, each UE 22-26 is assigned a uniqueset of Golay sequences, such as 4. For instance, user 0 is assigned foursequences, the top two sequences and the negation of those sequences. Byassigning each of the Golay sequences a virtual channel, upon reception,the receiving UE 22-26 determines the code of the downlink transmission.

A Golay sequence detector is shown in FIG. 9. The received signal iscorrelated with a Golay Correlator 123 and interleaved by an interleaver124 to detect the short codes. The arrangement of the short codes fortwo assigned sequences within the long codes is shown as Signature 0 and1. Using mixers 125, 126, the signatures are mixed with the detectedshort codes. Adders 127, 128 are coupled to the mixer and also to delaydevices 129, 130. The delay devices 129, 130 take the output of theadders 127, 128 and feed them back to those adders 127, 128 forcorrelation with the next short sequence. The output of each adder 127,128 determines the Golay sequence of the received signal.

FIG. 10 illustrates an assignment scheme for a system using two timeslot multiplexing for the physical channels. In table 132, each of thesixteen different signatures is assigned a downlink code and one of twotime slots. The selected time slot is indicated by the transmittedidentifier.

1. A method for sending data in a spread spectrum code division multipleaccess communication system between a selected user equipment out of aplurality of user equipments and a base station, the system having acommon packet channel using a predetermined set of codes, the methodcomprising: transmitting from the selected user equipment a preamble inthe common packet channel; in response to receiving the transmittedpreamble at the base station, selecting a code, out of a plurality ofcodes associated with the common packet channel; transmitting from thebase station a combined collision signal and channel indicator, thechannel indicator indicating the selected code and the channel indicatorbeing associated with the transmitted preamble; receiving the combinedcollision signal and channel indicator at the selected user equipment;and communicating data between the selected user equipment and the basestation using the selected code.
 2. The method of claim 1 wherein thecombined collision signal and channel indicator distinguishes betweencodes using an inversion.
 3. The method of claim 1 wherein thetransmitted signature is selected from a set of sixteen signatures andthe preamble is sent in one of eight slots.
 4. The method of claim 1wherein the selected code is used to define a downlink physical channelfor downlink communications.
 5. A base station for a spread spectrumcode division multiple access system comprising: a receiver forreceiving a preamble and packet data over a common packet channel, thecommon packet channel using a predetermined set of codes; a codeselection controller associated with said receiver for selecting a codefrom a plurality of codes; and a transmitter associated with saidcontroller which transmits to the user equipment a combined collisionsignal and channel indicator, the channel indicator indicating theselected code and the channel indicator being associated with thereceived preamble, whereby said transmitter sends communications to theuser equipment encoded with said selected code.
 6. The base station ofclaim 5 wherein the combined collision signal and channel indicatordistinguishes between codes using an inversion.
 7. A spread spectrumcode division multiple access communication system having a base stationand a plurality of user equipments, the system using a common packetchannel for communication, the common packet channel using apredetermined set of codes, the system comprising: the plurality of userequipments, each having: means for transmitting a preamble; means forreceiving a combined collision signal and channel indicator, the channelindicator indicating a selected code and the channel indicator beingassociated with the transmitted preamble; and means for communicatingwith the base station using the selected code; and the base stationhaving: means, in response to receiving the transmitted preamble forselecting the selected code out of a plurality of codes; and means fortransmitting the combined collision signal and channel indicator.
 8. Thesystem of claim 7 wherein the combined collision signal and channelindicator distinguishes between codes using an inversion.
 9. The systemof claim 7 wherein the transmitted signature is selected from a set ofsixteen signatures and the preamble is sent in one out of eight slots.10. The system of claim 7 wherein the selected code is used to define adownlink physical channel for downlink communications.